Heat reflecting material and heating device using the material

ABSTRACT

A heating apparatus  20  comprises a container  2  having a work housing space  1  formed therein, and a heat source  3  for heating a work W in the work housing space  1 . The apparatus further comprises a heat ray reflecting member  10  having a heat reflecting surface  10   a  thereof composed of a heat ray reflecting material, so as to allow the heat ray generated in the work housing space  1  to reflect on the heat reflecting surface  10   a  to thereby change the direction thereof towards the works W. The heat reflecting material is provided for reflecting heat ray in a specific wavelength band, comprises a stack of a plurality of element reflecting layers comprising materials having transparent properties to the heat ray, in which in the element reflecting layers, two adjacent layers are composed of a combination of materials differed from each other in refractive indices to the heat ray, while keeping difference between the refractive indices of 1.1 or larger.

TECHNICAL FIELD

[0001] This invention relates to a heat ray reflecting material capableof efficiently reflecting heat ray radiated from an exothermic body, anda heating apparatus using the same.

BACKGROUND ART

[0002] Manufacturing process for semiconductor wafers and devicefabrication process using the semiconductor wafers include processes inwhich the semiconductor wafers are heated to several-hundred tothousand-and-several-hundred degree centigrade, and for which a varioustypes of annealing furnaces such as those based on resistance heatingsystem (heater heating system), lamp heating system and so forth areused depending on purposes.

[0003] For example, in order to manufacture a silicon single crystalwafer, which is a representative semiconductor wafer, a silicon singlecrystal ingot is pulled using a single crystal pulling apparatus. In thepulling of the single crystal ingot, a quartz crucible in whichpolysilicon is charged is surrounded by a heater, and the crucible isheated to a temperature as high as 1,420° C. so as to fuse thepolysilicon as a source material. After the silicon single crystal thusproduced is processed to obtain the wafers, removal of damage, diffusionof impurities, vapor phase growth of semiconductor films and so forthare also carried out under heated atmosphere, where a variety of heatingapparatuses are used therefor. Also in the field of compoundsemiconductor, heating apparatuses are used for vapor phase growth orliquid phase growth of semiconductor films, and other annealingprocesses.

[0004] In order to raise the heating efficiency, the annealingapparatuses used for the above-described annealing are generallyconfigured so that heat insulating materials are disposed around theexothermic body such as the aforementioned heater or lamp so as toprevent heat from dissipating to the externals. In more simplifiedheating apparatuses, the heat insulating material may sometimes omittedfor size reduction.

[0005] Disposition of the heat insulating material, however, not onlyincreases the size of the heating apparatus but also needs a longer timeadditionally for heating of the heat insulating material because of alarge heat capacity thereof, and still also for cooling after completionof the annealing. Disposition of a forced cooling apparatus based onwater cooling, air cooling or the like further increases the size of theapparatus. Moreover, heat absorbed by the heat insulating material is ofcourse less contributable to heating of the works, and this worsens theenergy efficiency. It is a matter of course that the apparatus using noheat insulating material further suffers from more waste dissipation ofthe energy.

[0006] A subject of this invention is therefore to provide a heat rayreflecting material capable of reflecting heat ray emitted from anexothermic body in an extremely efficient manner, and a heatingapparatus capable of efficiently raising or lowering the temperature byconcentrating heat ray emitted from an exothermic body towards the worksby using the heat ray reflecting material.

DISCLOSURE OF THE INVENTION

[0007] To solve the aforementioned subject, a heat ray reflectingmaterial of the invention is such as being capable of reflecting heatray in a specific wavelength band, being a stack of a plurality ofelement reflecting layers comprising materials having transparentproperties to the heat ray, wherein, in the element reflecting layers,two adjacent layers are composed of a combination of materials differedfrom each other in refractive indices to the heat ray, while keepingdifference between the refractive indices of 1.1 or larger.

[0008] The heat ray reflecting material of the invention is configuredbased on a combination of the element reflecting layers respectivelyhaving transparent properties to the heat ray, differed from each otherin refractive indices to the heat ray, and keeping difference betweenthe refractive indices of 1.1 or larger. By composing the heat rayreflecting material based on the combination of the element reflectinglayers ensuring a large difference in the refractive indicestherebetween, the heat ray can be reflected at an extremely highreflectivity. Because a high reflectivity can be achieved only by alimited range of increase in the number of stacking of the elementreflecting layers, the heat ray reflecting material can be manufacturedat low costs. Difference in the refractive indices less than 1.1inevitably lowers the reflectivity, and increase in the number of cyclesof the stacking intended for an improved reflectivity raises the costs.The difference in the refractive indices between the element reflectinglayers to be combined is preferably kept at 1.2 or more, more preferably1.5 or more, and still more preferably 2.0 or more.

[0009] While “having transparent properties” herein can be defined as astatus that an object has a property of allowing electromagnetic wavesuch as light to pass therethrough, transparent property in thisinvention is preferably such as ensuring a transmissivity of the heatray to be reflected of as large as 80% or more for the thickness to beadopted. The transmissivity less than 80% may increase the absorptionratio of the heat ray, and may prevent the heat ray reflecting materialof the invention from fully exhibiting the effect of reflecting the heatray. The transmissivity is preferably 90% or more, and more preferably100%. A transmissivity of 100% herein means a transmissivity which canbe considered as almost 100% within a range of measurement limit (e.g.,within ±1% error) in general methods of measuring transmissivity.

[0010] Next, a heating apparatus according to a first aspect of thisinvention comprises a container having a work housing space formedtherein; a heat source for heating a work in the work housing space; anda heat ray reflecting member having a heat reflecting surface thereofcomposed of the heat ray reflecting material of this invention, so as toallow the heat ray generated in the work housing space to reflect on theheat reflecting surface to thereby change the direction thereof towardsthe work.

[0011] A heating apparatus according to a second aspect of thisinvention comprises at least an annealing chamber for carrying outannealing; an exothermic body disposed outside the annealing chamber;and a heat ray reflecting member surrounding the exothermic body and theannealing chamber and having a heat reflecting surface thereof composedof the heat ray reflecting material of this invention.

[0012] By applying the heat ray reflecting member composed of the heatray reflecting material of this invention to a heating apparatus, and byusing the member as a substitute, for example, for a part of or entireportion of the heat insulating material, delay in the heating or coolingspeed of the annealing apparatus ascribable to heat capacity of the heatinsulating material can be improved, and this enables more rapid heatingand cooling as compared with those for the conventional apparatus. It isalso possible to expand a length of uniform heating as compared withthat for the conventional annealing apparatus. It is still also possibleto achieve down-sizing and energy-saving effects of the apparatusbecause energy of the heat ray from the exothermic body can efficientlybe concentrated to the work.

[0013] The specific wavelength band of the heat ray selected from arange of 1 to 10 μm can cover wavelength ranges of heat ray necessaryfor heat processing in various applications, and can promise the effectof this invention. Although applicable fields of the heating apparatusof this invention are not specifically limited, one possible example islike the followings. In order to manufacture a silicon single crystalwafer which is a representative semiconductor wafer, first a siliconsingle crystal ingot is pulled using a single crystal pulling apparatus.In the pulling of the single crystal ingot, a quartz crucible in whichpolysilicon is charged is surrounded by a heater, and the crucible isheated to a temperature as high as 1,420° C. or above. The invention isapplicable to this heating apparatus. In this case, the wavelength bandof the heat ray to be reflected falls within a range from 1 to 5 μm, andmore preferably 1 to 3 μm, which successfully covers an essentialportion of spectra of the heat ray emitted from a melt of semiconductorsource material or from the heater for keeping a molten state of thesemiconductor source material. This makes it possible to efficientlyreflect and control the radiated infrared ray.

[0014] After the silicon single crystal thus produced is processed toobtain the wafers, removal of damage, diffusion of impurities, vaporphase growth of semiconductor films and so forth are carried out alsounder heated atmosphere as high as 400 to 1,400° C., for example, wherethe invention is applicable also to various heating apparatuses forcarrying out these processes. In these cases, the wavelength band of theheat ray to be reflected falls within a range from 1 to 30 μm, and morepreferably 1 to 10 μm. This makes it possible to efficiently reflect andcontrol the heat ray from a heat source such as the heater or the like,or from the work per se heated therewith.

[0015] On the other hand, heating apparatuses are used also in the fieldof compound semiconductor for vapor phase growth or liquid phase growthof semiconductor films, or other annealing processes (temperature range:400 to 1,400° C. or around), where the invention is again applicablethereto. The wavelength band of the heat ray to be reflected fallswithin a range from 1 to 30 μm, and more preferably 1 to 10 μm.

[0016] Moreover, heat processing is applied not only for theaforementioned semiconductor materials, but is widely applied to variousmaterials or to a vast variety of processes. For example, variousheating apparatuses are used in the manufacture of metal materials ormetal members, for melting of the materials, sintering, hot working andother annealing processes (temperature range: 400 to 1,800° C. oraround, wavelength band of heat ray to be reflected: 0.3 to 30 μm). Alsoin the manufacture of inorganic materials such as ceramics or glasses,heating apparatuses are used for sintering, processing or otherannealing processes (temperature range: 700 to 1,800° C. or around,wavelength band of heat ray to be reflected: 0.3 to 20 μm). Besidesthese, drying furnace or the like used for various applications can besaid as a kind of heating apparatus. Further besides those of industrialuse, heating cooking instruments (e.g., oven) for business use or homeuse can be exemplified. These are used at a relatively low temperaturerange, for example, 200 to 500° C. or around (wavelength band of heatray to be reflected: 3 to 40 μm).

[0017] The stack composing the heat ray reflecting member can beconfigured so as to include first and second element reflecting layersdiffered in refractive indices and being adjacent to each other, and sothat stack cycle units, each of which comprising a first and a secondelement reflecting layers, are formed in two or more cycles on thesurface of a base member. The periodic changes in the refractive indexof the stack in the thickness-wise direction thereof is successful infurther raising the heat ray reflectivity. In this case, a largerdifference in the refractive indices of the plurality of materialscomposing the stack cycle units can result in a larger reflectivity. Forexample, the simplest constitution of the stack cycle unit relates to atwo-layered structure comprising the first element reflecting layer andthe second element reflecting layer differed in the refractive indicesto the heat ray from each other. In this case, a larger differencebetween the refractive indices of both layers can reduce the number ofthe stack cycle units necessary for keeping the reflectivity of heat rayat a sufficiently high level. The number of layers composing the stackcycle unit may be three or more.

[0018] The thickness of the stack cycle unit can be set smaller than thecentral wavelength of the heat ray to be reflected. This setting issuccessful in the formation of an optical stop band structure (orone-dimensional photonic band gap structure) against the heat ray of aspecific wavelength band in the thickness-wise direction of the heatreflecting layer depending on the distribution of refractive indices inthe stack cycle units, and this makes it possible to almost completelyreflect the heat ray of such specific wavelength band to thereby fullyextract the aforementioned effects of the invention. The thickness ofthe individual layers and the number of cycles can be determined bycalculations or experiments based on a range of the wavelength band tobe reflected. Adoption of the combination of the materials differed inthe refractive indices by 1.1 or more as in the invention is successfulin readily realizing the stack cycle structure having a heat rayreflectivity close to complete reflection with a relatively small numberof formation cycles of the stack cycle unit, or more specifically 5cycles or less. In particular, adoption of a combination ensuring adifference in the refractive indices of 1.5 or more is successful inrealizing a large heat ray reflectivity as described in the above onlywith the number of formation of cycles of 4 cycles, 3 cycles or even assmall as 2 cycles.

[0019] The range of the wavelength band to be reflected depends on thetemperature of the heat source. That is, of the radiated energiesemitted from a unit area of object surface within a unit time under apredetermined temperature, the maximum limit energy is shown bymonochromatic emissive power obtained from a perfect black body. Thiscan be expressed by the equation below (Planck's Law):

E _(bλ) =Aλ ⁻⁵(e ^(B/λT)−1)⁻¹ [W/(μm)²]

[0020] where, E_(bλ): monochromatic emissive power of black body[W/(μm)²], λ: wavelength [μm], T: absolute temperature of object surface[K], A: 3.74041×10⁻¹⁶[W·m²], and B: 1.4388×10⁻²[m·K]. FIG. 8 is a graphshowing relations between monochromatic emissive power (E_(bλ)) andwavelength obtained when absolute temperature T of the object surfacewas varied. It is seen that the monochromatic emissive power peak lowersand shifts towards the longer wavelength side as T decreases.

[0021] Materials of the element reflecting layer composing the stack arepreferably selected from combinations of the materials which are stableagainst high temperatures and capable of ensuring anecessary-and-sufficient level of difference in the refractive indicestherebetween for infrared reflection. The stack can be configured so asto include a layer which comprises a semiconductor or an insulatorhaving a refractive index of 3 or above, as the first element reflectinglayer which serves as a high-refractive-index layer. Use of thesemiconductor or an insulator having a refractive index of 3 or above asthe first element reflecting layer facilitates to ensure a largedifference in the refractive index from that of the second elementreflecting layer to be combined therewith. Table 1 summarizes refractiveindices of materials for the element reflecting layers applicable to theinvention. Substances having refractive indices of 3 or above can beexemplified by Si, Ge and 6h-SiC, and also by compound semiconductorssuch as Sb₂S₃, BP, AlP, AlAs, AlSb, GaP and ZnTe. As for semiconductorsand insulators, it is preferable to use those having band gap energiessufficiently larger (e.g., 2 eV or above) than photon energies of theheat ray, because those of the direct transition type having band gapenergies close to the photon energies of the heat ray to be reflectedtend to absorb the heat ray. On the other hand, those of indirecttransition type (e.g., Si, Ge) can suppress the heat ray absorption at alow level and applicable to the invention in a desirable manner even ifthe band gap energies thereof are smaller than the above-describedvalue. Among others, Si is advantageously used for the invention becauseit has a relatively low price, can readily be made in a form of a thinfilm, and has a refractive index of as large as 3.5. Using a Si layer asthe first element reflecting layer can therefore realize a stackedstructure having a high reflectivity at low costs.

[0022] Next, low-refractive-index materials for composing the secondelement reflecting layer can be exemplified by SiO₂, BN, AlN, Al₂O₃,Si₃N₄ and CN. In this case, it is necessary to select the materials forthe second element reflecting layer so as to ensure a difference in therefractive index of 1.1 or more depending on the types of the materialsalready selected for the first element reflecting layer. Table 1 belowsummarizes refractive indices of these materials. Among others, adoptionof SiO₂ layer, BN layer or Si₃N₄ layer is particularly advantageous inensuring a large difference in the refractive indices. The SiO₂ layerhas a refractive index of as small as 1.5, and can ensure a largedifference from that of the first element reflecting layer typicallycomposed of a Si layer. It is also advantageous in that it can readilybe formed typically by oxidation of the Si layer. On the other hand, theBN layer has a refractive index ranging from 1.65 to 2.1 depending onthe crystal structure or orientation. The Si₃N₄ layer has a refractiveindex ranging from 1.6 to 2.1 depending on quality of the film. Althoughthese layers have slightly larger refractive indices as compared withthat of SiO₂, they can still ensure a large difference of refractiveindex as much as 1.4 to 1.85 from that of Si. Considering thetemperature range (400 to 1,400° C.) generally used for manufacturing ofsilicon wafers, it is effective, in view of efficiently reflecting theradiated heat, to compose the heat ray reflecting layer so as toessentially include a Si layer and additionally include at least eitherof a SiO₂ layer or a BN layer, and more specifically to compose it sothat the Si layer and SiO₂ layer and/or the BN layer are included as theelement reflecting layer. The BN layer is desirable when applied toultra-high-temperature use since it has a melting point considerablyhigher than that of SiO₂. BN is also advantageous in that it can emitonly N₂ as an outgas if decomposed at high temperatures, where boronremains in the surface in a semi-metallic state, and is thus notaffective to electric characteristics of the semiconductor wafers suchas Si wafer and the like. Table 2 shows exemplary combination ofpreferable materials by temperature zones. TABLE 1 Substance Refractiveindex (h) Substance Refractive index (h) Si 3.5 c-BN 2.1 6h-SiC 3.2 h-BN1.65 (// c-axis) 3c-SiC 2.7 2.1 (⊥ c-axis) Diamond 2.5 Al₂O₃ 1.8 TiO₂2.5 SiO₂ 1.5 AlN 2.2 Sb₂S₃ 4.5 Si₃N₄ 2.1 Refractive Indices ofSemiconductors Width of Refractive index forbidden band Transition nCompound [eV] 300 K type (hv ≅ Eg) Si 1.2 indirect 3.4 Ge 0.7 indirect4.0 6h-SiC 3.2 indirect 2.7 h-BN 2.1 BP 2.0 indirect 3.5 AlN 6.2 2.2 AlP2.4 indirect 3.0 AlAs 2.2 indirect 3.2 AlSb 1.6 indirect 3.4 GaN 3.4direct 2.2 GaP 2.3 indirect 3.5 ZnS 3.8 direct 2.5 ZnSe 2.7 direct 2.6ZnTe 2.3 direct 3.2 CdS 2.4 direct 2.5

[0023] TABLE 2 Layers composing Application periodic structureLow-to-middle temperature use (<1,100° C.) Si, SiO₂ High-temperature use(1,100-1,400° C.) Si, BN Ultra-high-temperature use (1,400-1,600° C.)SiC, BN

[0024] Next paragraphs will describe results of calculative examinationon conditions which can ensure an almost complete reflection of infraredregion by forming an one-dimensional photonic band gap structure usingSi and SiO₂. Si has a refractive index of approximately 3.5, and a thinfilm thereof is transparent to light in the infrared region having awavelength of approximately 1.1 to 10 μm. SiO₂ has a refractive index ofapproximately 1.5, and a thin film thereof is transparent to light inthe infrared region having a wavelength of approximately 0.2 to 8 μm(visible to infrared regions). FIG. 1 is a sectional view of a heat rayreflecting layer having formed on Si substrate 100, four cycles of thestack cycle units, each of which comprising two layers of a Si layer “A”of 100 nm thick and a SiO₂ layer “B” of 233 nm thick. This structure canachieve a reflectivity of infrared radiation in the 1 to 2 μm band ofnearly 100% as shown in FIG. 2, and can inhibit transmission of theinfrared radiation. It is also allowable to compose the base member withother materials (e.g., quartz (SiO₂)), another Si layer is formedthereon, and stack cycle unit similarly comprising two layers of a Silayer “A” and SiO₂ layer “B” can be formed further thereon. For example,a heat source of 1,600° C. has a maximum intensity in 1 to 2 μm band,where coverage of this band together with 2 to 3 μm band (correspondingto a peak wavelength region of heat ray spectrum obtained from a heatsource of around 1,000 to 1,200° C.) is accomplished by adding anotherperiodic combination differed in the wavelength region to be reflected.That is, an allowable configuration may be such as shown in FIG. 3,where the aforementioned combinations of 100 nm (Si)/233 nm (SiO₂) areadded with another thickened combinations of 157 nm (Si)/366 nm (SiO₂)(A′/B′ in FIG. 3).

[0025] In contrast to that the above-described 4-cycle structure of 100nm (Si)/233 nm (SiO₂) ensures nearly 100% reflectivity for infraredradiation in 1 to 2 μm band, the 4-cycle structure of 157 nm (Si)/366 nm(SiO₂) ensures nearly 100% reflectivity for infrared radiation in 2 to 3μm band, as shown in FIG. 4. Therefore the structure shown in FIG. 3 inwhich these structures are stacked can provide a material which canensure nearly 100% reflectivity over 1 to 3 μm band.

[0026] Similarly, 3 to 4.5 μm band can be covered by properly selectinga further thickened combination of Si layer and SiO₂ layer and byforming the 4-cycle structure. Combination of layer only capable ofensuring a difference between the refractive indices smaller than thatbetween Si and SiO₂ may need a larger number of cycles, so that a largerdifference between two layers to be selected is more advantageous.According to the above-described combination, a total thickness of 1.3gm ensures almost complete reflection of 1 to 2 μm band, and a totalthickness of 3.4 gm ensures that of 1 to 3 μm band.

[0027] On the other hand, FIG. 5 shows a calculated result of thereflectivity of the heat ray reflecting layer having a 4-cycle structureof 94 nm (SiC)/182 nm (BN), based on selection of 6h-SiC (refractiveindex 3.2) and h-BN (refractive index 1.65) capable of ensuring a largedifference in the refractive indices therebetween similarly to the caseof Si and SiO₂. It is known in this case that reflectivity of nearly100% is achieved for light (heat ray) in 1 to 1.5 μm band.

BRIEF DESCRIPTION OF DRAWINGS

[0028]FIG. 1 is a sectional view of a heat ray reflecting material ofthe invention, having a 4-cycle structure of Si layers and SiO₂ layers;

[0029]FIG. 2 is a chart showing a heat ray reflectivity characteristicof the heat ray reflecting material having the structure shown in FIG.1;

[0030]FIG. 3 is a sectional view of a heat ray reflecting materialhaving a structure in which the 4-cycle structure shown in FIG. 1 isstacked with another 4-cycle structure of Si layers and SiO₂ layershaving different thickness;

[0031]FIG. 4 is a chart showing a heat ray reflectivity characteristicof the heat ray reflecting material having the structure shown in FIG.3;

[0032]FIG. 5 is a chart showing a heat ray reflectivity characteristicof the heat ray reflecting material of the invention, having a 4-cyclestructure of 6h-SiC layers and h-BN layers;

[0033]FIG. 6 is a drawing of a manufacturing process flow of the heatray reflecting material having a periodic structure of the invention;

[0034]FIG. 7A is a longitudinal sectional view of a heating apparatusaccording to a first applied example of the heat ray reflecting materialof the invention;

[0035]FIG. 7B is a transverse sectional view of the heating apparatusaccording to the first applied example of the heat ray reflectingmaterial of the invention;

[0036]FIG. 8 is a graph showing relations between monochromatic emissivepower of black body (E_(bλ)) and wavelength when absolute temperature Tof object surface is varied;

[0037]FIG. 9 is a drawing of a heating apparatus according to a secondapplied example of the heat ray reflecting material of the invention;

[0038]FIG. 10 is a spectral chart showing a difference spectrum of theheat ray reflecting material and a reference in an embodiment of theinvention;

[0039]FIG. 11 is a drawing of a heating apparatus according to a thirdapplied example of the heat ray reflecting material of the invention;

[0040]FIG. 12 is a drawing of a heating apparatus according to a fourthapplied example of the heat ray reflecting material of the invention;and

[0041]FIG. 13 is a drawing of a heating apparatus according to a fifthapplied example of the heat ray reflecting material of the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0042] The following paragraphs will describe best modes for carryingout the invention, where the invention is by no means limited thereto.FIG. 6 shows a manufacturing process flow of a heat ray reflectingmaterial 20 of the invention. First, a material for composing a basemember 23 for the heat ray reflective material is selected, and thematerial is then processed to obtain a necessary form (process step (a)in FIG. 6).

[0043] The form required herein for the base member may differ byapplications of the heat ray reflecting material, and for the caseintended for application to a heating apparatus (e.g.,oxidation/diffusion furnace for annealing) 20 for semiconductor wafersas shown in FIGS. 7A and 7B, the base member is processed to obtain aform capable of surrounding a resistance exothermic body (heat source) 3disposed outside a reaction tube (container) 2 having an annealingchamber (work housing space) 1. A heat ray reflecting member 10 havingthe heat ray reflecting material of the invention formed on the basemember is, therefore, configured so as to surround the exothermic body 3and the annealing chamber 1, and so that the surface thereof is composedof the heat ray reflecting material of the invention. Thus-configuredheating apparatus 20 is such as having the reaction tube (container) 2having the housing space 1 for housing work W formed therein; theexothermic body (heat source) 3 for heating a work (semiconductor wafer)W in the work housing space 1; and the heat ray reflecting member 10having a heat reflecting surface 10 a thereof composed of the heat rayreflecting material of the invention, so as to allow the heat raygenerated in the work housing space to reflect on the heat reflectingsurface 10 a to thereby change the direction thereof towards the work(semiconductor wafer) W.

[0044] Any large heat conduction from the exothermic body 3 herein mayraise a problem of temperature rise of the heat ray reflecting member 10and deformation of the base member. In this case, it is preferable toprovide a cooling mechanism for cooling the heat ray reflecting member10. While the cooling mechanism may be of air-cooled type such as havinga radiating member such as a fin, or such as making contact with coolair flow, this embodiment adopts a cooling mechanism of water-cooledtype higher in the cooling efficiency. More specifically, as indicatedby a dashed line in FIG. 7A, a structure adopted herein includes awater-cooled wall 11, having a water-cooling pathway 12 (composed of apipe member for circulating water or a water jacket) embedded therein,and formed so as to contact with the heat ray reflecting member 10.While the heat ray reflecting member 10 and water-cooled wall 11 in thisembodiment are formed as separate members, it is also possible to embedthe water cooling pathway 12 in the base member of the heat rayreflecting member 10, to thereby allow the heat ray reflecting member 10per se to have functions of the cooling mechanism.

[0045] The base member 23 shown in FIG. 6 preferably has a mechanicalstrength and heat resistance, where Si, SiO₂, SiC, BN and the like aresuitable as the materials therefor. These materials are used forsubstrates on which semiconductor devices are fabricated, and typicallyfor the reaction tube of general annealing apparatuses or annealing jigsor the like for annealing the substrates, have a wide versatility, andcan be processed to obtain various forms.

[0046] Next, a first element reflecting layer “B” which is transparentto the heat ray emitted from the exothermic body, is formed on thesurface of the base member 23 (process step (b) in FIG. 6). Thereafter,the second element reflecting layer “A”, having a refractive indexdifferent from that of the first element reflecting layer “B” is formedon the surface of the first element reflecting layer “B” (process step(c) in FIG. 6). While there are no special limitations on the method offorming these layers, the CVD process can form a variety of layers suchas Si, SiO₂, SiC, BN, Si₃N₄ and so forth. When the base member 23 is aSi substrate, the first layer of SiO₂ layer as the first elementreflecting layer can be formed by thermal oxidation. Similarly, for thecase where a Si layer is used as the first or second element reflectinglayer, a SiO₂ layer as the other element reflecting layer can be formedagain by the thermal oxidation.

[0047] A periodic structure 24 is then produced by forming two or morecycles of these first and second element reflecting layers, to therebyform the heat ray reflecting material 20 of the invention (process step(d) in FIG. 6). These two layers having a periodicity may be formed onlyon one surface or on both surfaces of the base member. The thickness andthe number of cycles can be determined by calculations or experimentsbased on the range of wavelength band to be reflected, as can beunderstood from the above-described exemplary case of SiO₂ and Si. Therange of wavelength band to be reflected depends on temperature of theexothermic body.

[0048] The heat ray reflecting material of the invention can inseparablybe incorporated into the annealing apparatus per se, or can be used as adummy wafer during the annealing proceeded in a conventional annealingapparatus. That is, as shown in FIG. 9, it is allowable to make heat rayreflective materials 30 of the invention in a wafer form having analmost equal shape with the semiconductor wafers W to be annealed orhaving a larger diameter, and to dispose them on the front and rear endsof the semiconductor wafers W aligned on an annealing boat, by which theheat ray can be reflected towards both sides of the semiconductor wafersand is prevented from dissipating, and therefore the semiconductor waferW can efficiently be annealed.

[0049] Another embodiment will be explained below.

[0050]FIG. 11 shows a heating apparatus 40 used for sintering orannealing of the works W comprising a metal member or a ceramic member,where the apparatus has resistance exothermic elements 41 as heatsources inside a furnace wall member 42 composing the container. On theinner surface of the furnace wall member 42, heat ray reflectingmaterials 43 having the periodic structure similar to that described inthe above are disposed in the space not occupied by the resistanceexothermic elements 41. The heat ray reflecting materials 43 can notonly concentrate the heat ray from the resistance exothermic elements 41to the works W, but can also reflect radiated heat from the heated worksW back to the works W, to thereby ensure a more efficient heating.

[0051]FIG. 12 shows a heating apparatus 50 using combustion heat sources51 such as gas burners, where the apparatus is configured so as todispose heat sources 51 typically at the bottom of a furnace body 52which composes the container, and so as to allow the generated heatstream HS to circulate by convection within the furnace body 52 tothereby heat the works W. A heat ray reflecting material 56 of theinvention formed on a base member 55 is disposed so as to surround theworks W while keeping an aperture path AP for the convection opened.Reflection of the radiated heat from the heated works W back to theworks W ensures a more efficient heating. A reference numeral 53represents an exhaustion path for combustion gas.

[0052] Besides the above-described applications which use exothermicbodies of relatively high temperatures such as those used inmanufacturing processes for semiconductors or metal materials, the heatray reflecting material of the invention is also applicable toexothermic bodies of relatively low temperatures of several hundreds tothousand degrees centigrade. In this case, the base member on which theheat ray reflecting material is formed or a layer for composing theperiodic structure may comprise a glass, paint, plastic or gas such asair.

[0053]FIG. 13 shows a heating apparatus 60 for cooking foods appliedwith the invention. Heat sources 61 individually composed of aresistance heating wire or a ceramic heater are disposed on the innersurface of a container 62 having a door 65 though which a food F is putin or taken out, and a heat ray reflecting material 64 formed on a basemember 63 is disposed while keeping a positional relation notinterfering the heat ray directed from the heat sources 61 towards thefood F (in spaces between the adjacent heat sources 61, 61 herein). Theheat ray reflecting material 64 is also disposed on the inner surface ofthe door 65. This configuration makes it possible to efficientlyconcentrate the heat to the food F, and can realize a cooking heatingapparatus having a low power consumption and high output power. Theapparatus can heat the food F from all directions and allow the heat rayto reach deep into the food F in an efficient manner, by which a thickmeat or the like can be cooked without causing non-uniformity. It isconvenient herein to configure a part of the door 65 using a transparentbase member such as a heat-resistant glass or the like, and to formthereon the heat ray reflecting material using Si/SiO₂, which aretransparent to the visible light, because the food F during cooking canbe observed.

EXAMPLE

[0054] The following paragraphs will describe results of experimentscarried out to confirm the effects of the invention.

[0055] On a silicon wafer having a diameter of 150 mm, a thermal oxidefilm of 233 nm thick was formed by dry oxidation at 1,000° C. Further onthe thermal oxide film, a polysilicon layer of 205 nm thick was thendeposited by the reduced-pressure CVD process. The thermal oxidation wasrepeated again to thereby form a thermal oxide film of 233 nm thickwhile leaving the polysilicon layer of 100 nm thick.

[0056] Thereafter, formation of the 205-nm-thick polysilicon layer andthe 233-nm-thick thermal oxide layer were repeated twice, and apolysilicon layer of 100 nm thick was finally formed to thereby form the4-cycle structure of polysilicon layer/thermal oxide film as shown inFIG. 1. The structure was formed on both surfaces of the wafer for theconvenience of the process.

[0057] The wafer was irradiated with infrared radiation, and anabsorption spectrum was obtained by measuring the transmitted light. Forreference, an absorption spectrum of a silicon wafer having noperiodic-structured layers formed thereon was measured. Difference ofthese spectra was obtained and shown in FIG. 10. It is found from theresults shown in FIG. 10 that the absorptivity becomes high in awavelength band ranging from approximately 1 to 2 μm (1,000 to 2,000nm). This is ascribable to increase in the reflectivity in the 1 to 2 μmwavelength band due to the periodic structure on the wafer surface, andconsequent decrease in the transmissivity of light in that wavelengthband, so that the spectrum obtained in the above was such as apparentlyshowing that the absorptivity in that wavelength band increased. Thismeans that the wafer of the invention shows an extremely higherreflectivity of infrared radiation in the wavelength band ofapproximately 1 to 2 μm than the reference shows. This shows a goodcoincidence with the calculated result shown in FIG. 2.

1. A heat ray reflecting material capable of reflecting heat ray in aspecific wavelength band, being a stack of a plurality of elementreflecting layers comprising materials having transparent properties tothe heat ray, wherein, in the element reflecting layers, two adjacentlayers are composed of a combination of materials differed from eachother in refractive indices to the heat ray, while keeping differencebetween the refractive indices of 1.1 or larger.
 2. The heat rayreflecting material as claimed in claim 1 or 2, wherein the specificwavelength band to be reflected by the heat ray reflecting materialfalls within a range from 1 to 10 μm.
 3. The heat ray reflectingmaterial as claimed in claim 1 or 2, wherein the stack comprises firstand second element reflecting layers differed in refractive indices andbeing adjacent to each other, and is configured so that stack cycleunits, each of which comprising a first and a second element reflectinglayers, are formed in two or more cycles on the surface of a basemember.
 4. The heat ray reflecting material as claimed in claim 3,wherein the stack includes a layer which comprises a semiconductor or aninsulator having a refractive index of 3 or above, as the first elementreflecting layer.
 5. The heat ray reflecting material as claimed inclaim 4, wherein the first element reflecting layer is a Si layer. 6.The heat ray reflecting material as claimed in claim 4 or 5, wherein thestack includes a layer comprising any one of SiO₂, BN, AlN, Si₃N₄,Al₂O₃, TiO₂, TiN and CN, as the second element reflecting layer.
 7. Theheat ray reflecting material as claimed in claim 3, wherein the first orsecond element reflecting layer is a Si layer, and the other elementreflecting layer adjacent thereto is a SiO₂ layer or a BN layer.
 8. Theheat ray reflecting material as claimed in any one of claims 3 to 7,wherein the number of formation cycles of the stack cycle unit is 5cycles or less.
 9. The heat ray reflecting material as claimed in anyone of claims 3 to 8, wherein the base member comprises any one of Si,SiO₂, SiC, BN, AlN, Si₃N₄, Al₂O₃, TiO₂, TiN and CN.
 10. A heatingapparatus comprising: a container having a work housing space formedtherein; a heat source for heating a work in the work housing space; anda heat ray reflecting member having a heat reflecting surface thereofcomposed of the heat ray reflecting material as claimed in any one ofclaims 1 to 9, so as to allow the heat ray generated in the work housingspace to reflect on the heat reflecting surface to thereby change thedirection thereof towards the work.
 11. A heating apparatus comprisingat least: an annealing chamber for carrying out annealing; an exothermicbody disposed outside the annealing chamber; and a heat ray reflectingmember surrounding the exothermic body and the annealing chamber andhaving a heat reflecting surface thereof composed of the heat rayreflecting material as claimed in any one of claims 1 to 8.